Wavelength division multiplexed (WDM) optical communication systems are known in which multiple optical signals or channels, each having a different wavelength, are combined onto an optical fiber. Such systems typically include a laser associated with each wavelength, a modulator configured to modulate the optical signal output from the laser, and an optical combiner to combine each of the modulated optical signals. Such components are typically provided at a transmit end of the WDM optical communication system to transmit the optical signals onto the optical fiber. At a receive end of the WDM optical communication system, the optical signals are often separated and converted to corresponding electrical signals that are then processed further.
Known WDM optical communication systems are capable of multiplexing 40 channels at 100 GHz spacing or 80 channels at 50 GHz spacing. These WDM optical communication systems occupy an overall bandwidth of 4000 GHz. At 50 GHz channel spacing and 100 GHz channel spacing, the occupied optical fiber bandwidth or spectrum is not efficiently used. As rapid growth of the Internet continues, and new applications arise, there is an increasing demand for higher data rates provided by underlying networks, which may be supported by advances in optical communication systems. Due to the increased demand, the information carrying capacity of an optical fiber preferably should also increase. As used herein, the terms “carrier, “channel” and “optical signal” may be used interchangeably.
One method to increase the data capacity of the occupied optical fiber bandwidth is to employ higher data rate modulation formats to modulate the optical signals or channels to carry data at higher rates. Such higher rate modulation formats, however, are typically more susceptible to noise, and, therefore, may not be used in transmission of optical signals over relatively long distances. Thus, the modulation format must be chosen according to a desired reach, or distance, the transmitted channels are expected to span. Other known systems, commonly called dense wavelength-division multiplexing systems (DWDM), are capable of increasing the total data capacity by packing even more densely, additional channels on an optical fiber by more closely spacing the channels together, such as at 25 GHz spacing between channels. While 25 GHz channel spacing is an improvement over 50 GHz and 100 GHz spacing, further improvement is still needed to meet the demands of increased data rates.
Conventional DWDM systems for optical communications typically conform to a wavelength or frequency grid defined by the International Telecommunications Union (ITU). The most common frequency grid is that used for channel spacing at wavelengths around 1550 nm as defined by ITU-T G.694.1 (2002). The ITU grid is defined relative to 193.1 THz and extends from 191.7 THz to 196.1 THz with 100 GHz periodic spacing between adjacent channels. Recently, however, as optical technology has improved, the frequency grid has practically been extended to cover 186 THz to 201 THz and is sub-divided to provide the 50 GHz and 25 GHz spaced channels discussed above. Because the ITU grid is an accepted standard, many optical components used in known optical communication systems have been developed and optimized to conform to the ITU defined frequency channels and their periodic spacing. However, conforming to such a restrictive frequency grid, while convenient, may undesirably limit the data carrying capacity of an optical communication system.
Preferably, the information carrying capacity of an optical communication system should be optimized to carry a maximum amount of data over a maximum length of optical fiber while efficiently utilizing the bandwidth supported by available optical components, such as optical amplifiers, for example. Accordingly, individual carrier or channel spacing should be minimized according to the available optical components and transmitter and receiver technology capable of reliably transmitting and receiving such minimally spaced channels. Therefore, a greater number of channels can be packed in a given bandwidth, resulting in more efficient use of network resources and the occupied optical spectrum of the channels. Accordingly, increased data demands of the network drive a need to provide a plurality of minimally spaced carriers to increase optical communication system network capacity. Additionally, unique customer requirements provide a need to flexibly group the plurality of minimally spaced carriers together in blocks or “superchannels” that can be individually routed throughout the network and that can be multiplexed with other blocks of similar minimally spaced carriers. Furthermore, in order to utilize the convenience of available optimized optical components and to simplify transmitter and receiver architecture, there is a need to configure such superchannels with periodic fixed spaced carriers.